[CALCITE-4366] LatticeSuggester should treat UNION and VALUES sub-expressions as tables

[julianhyde]

[vlsi]

@julianhyde , Could you please refrain from adding @Nonnull? It should be a default mode, so there's no point in adding @Nonnull.

It would be nice if you could annotate nullable (e.g. in case you expect something to receive or return null)